Split /etc/{,man}path by colons too

I don't know how /etc/manpath ends up containing lines like
"path1:path2".  But path_helper splits them so we should do too.

4ea11424b8/path_helper/path_helper.c (L149)

Fixes #10684

(cherry picked from commit 95d61ea0fb)

share/config.fish

@@ -166,13 +166,11 @@ if status --is-login
 
             # Populate path according to config files
             for path_file in $argv[2] $argv[3]/*
-                if test -f $path_file
-                    while read -l entry
-                        if not contains -- $entry $result
-                            test -n "$entry"
-                            and set -a result $entry
-                        end
-                    end <$path_file
+                for entry in (string split : <? $path_file)
+                    if not contains -- $entry $result
+                        test -n "$entry"
+                        and set -a result $entry
+                    end
                 end
             end